Write-once optical record carrier for high-speed recording

ABSTRACT

The present invention relates to a write-once optical record carrier for high speed recording, in particular to a DVD+R disc. Such a record carrier comprises in general at least a substrate layer ( 3 ), a recording layer ( 2 ) of an organic dye material on top of the substrate layer ( 3 ) and a metal reflective layer ( 1 ) on top of the recording layer ( 2 ). In order to obtain a less steep temperature gradient at the interface between the recording layer ( 2 ) and the reflective layer ( 1 ) and thus to prevent mechanical stress leading to a delamination problem it is proposed to reduce the thickness of the metal reflective layer ( 1 ) to a range below 75 nm. A dielectric layer of a thickness below 50 nm between the recording layer ( 2 ) and the metal reflective layer ( 1 ) is also enclosed.

The present invention relates to a write-once optical record carrier,such as a DVD+R, in particular to a single-layer DVD+R, which is adaptedfor high-speed recording of data thereon.

At present, the development of a high-speed DVD+R standard has a highpriority. Current dye-based high-speed DVD+R media exhibit reasonableperformance up to 4× or even 6×. The power margins, however, are gettingmore and more narrow at higher speeds. It is believed that at the highpowers required for high-speed recording delamination occurs at thedye-metal interface, i.e. at the interface between the recording layermade of an organic dye material and a metal reflective layer providedfor cooling of the recording layer. These problems raise concerns aboutthe possibility to achieve higher recording speeds with write-onceoptical record carriers, in particular DVD+R media, comprising arecording layer made of an organic dye material. However, the use ofdyes is considered favourable because of the backwards compatibility ofrecorded disc on existing players due to the dye's intrinsic hightransparency which allows (together with a reflector layer) a highreflectivity disc.

The thickness of the metal reflective layer is usually around 100 nm. Animportant effect of the presence of the metal reflective layer is itslarge cooling power, i.e. its high heat capacity. It is thus believedthat a reduction of the thickness of the metal reflective layer willresult in a less efficient cooling of the dye recording layer. Since thedye has only a very poor heat conduction, the efficient cooling by themetal reflective layer induces a large temperature gradient near theinterface between the recording layer and the metal reflective layer. Itis possible that mechanical stress that may result from this steepgradient leads to the above-mentioned delamination problem.

U.S. Pat. No. 5,718,961 discloses a phase-change type optical disc inwhich a first dielectric film, a second dielectric film, a recordingfilm and a reflective film are sequentially stacked on a substrate. Thethickness of the reflective layer can be in a broad range of 10 to 120nm. By use of ZnO—BN as material for the first and second dielectricfilm a high recording sensitivity and thermal stability can be achieved,and heat produced during recording can be rapidly dissipated to thereflective layer.

It is an object of the present invention to provide a write-once opticalrecord carrier which allows higher recording speeds but has a reducedtemperature gradient at the interface between the recording layer andthe metal reflective layer to avoid mechanical problems such asdelamination.

This object is achieved according to the present invention by awrite-once optical record carrier as claimed in claim 1 comprising:

a substrate layer,

a recording layer of an organic dye material on top of the substratelayer and

a metal reflective layer of a thickness below 75 nm on top of therecording layer.

The invention is based on the finding that, contrary to common believe,a reduction of the thickness of the metal reflective layer can beadvantageous. It has been found that by a reduction of the thickness,the cooling becomes less efficient, which, however, leads in consequenceto a temperature gradient at the interface between the recording layerand the metal reflective layer which is more gradual. This will reducemechanical stress and thereby prevent delamination. It has thus beenrecognized that a deterioration of the cooling in the recording stackmay actually improve the high-speed recording performance. This meansthat by this invention it is proposed to do the opposite from whatexperts in this field commonly believe both for record carriers having arecording layer made of an organic dye material or made of aphase-change material.

Preferred embodiments of the invention are defined in the dependentclaims. While an improvement of the high-speed recording performance canbe achieved by a metal reflective layer thickness below 75 nm, a furtherimprovement can be achieved by reducing the thickness even more below 50nm, in particular below 30 nm.

According to another preferred embodiment an additional dielectric layeris provided between the recording layer and the metal reflective layer.This dielectric layer acts as a thermal barrier and can mimic thereduced heat-sink capability of the reflective layer. The introductionof the additional dielectric layer, e.g. SiO₂, ZnS, ZnS-SiO₂ mixture(e.g. 8:2), TiO₂ or an other dielectric material, slightly enhances thereflection of the recording stack at the cost of reduced absorption.However, it can be foreseen further, to use a thinner recording layerwhich usually has a thickness of 100 nm, or a dye having a higher kvalue, k being the imaginary part of the complex refractive index, inorder to compensate this.

Optionally, another dielectric layer can be provided between therecording layer and the substrate layer, in particular to improvestability of the whole recording stack. At least the first dielectriclayer between the recording layer and the metal reflective layer has athickness below 50 nm, in particular below 25 nm.

A preferred material for the metal reflective layer substantiallyconsists of silver (Ag). However, other materials such as Al, Au orother metals can be used as well.

The invention will now be explained in more detail with reference to thedrawings, in which:

FIG. 1 shows power margins for a DVD+R disc with a 100 nm Ag layer;

FIG. 2 shows power margins for a DVD+R disc with a 10 nm Ag layer;

FIG. 3 a shows the temperature distribution after applying a DC-powerlevel to a conventional write-once record carrier;

FIG. 3 b shows the temperature distribution after applying a DC-powerlevel to a write-once optical record carrier according to the presentinvention;

FIG. 4 shows the thermal distribution for another embodiment of a recordcarrier according to the invention;

FIG. 5 shows the thermal distribution for still another embodiment of arecord carrier according to the invention;

FIG. 6 shows a schematic layout of a first embodiment of a recordcarrier according to the invention; and

FIG. 7 shows a schematic layout of a second embodiment of a recordcarrier according to the invention.

FIG. 1 shows a typical power margin in case of a thick reflective layer,in particular a 100 nm Ag layer, in a DVD+R disc. Shown is the jitter inpercentage over write power in mW for leading and trailing edges of bitsto be recorded on the disc. This power margin needs to be compared tothe power margin for a thin reflective layer shown in FIG. 2 where thereflective layer made of Ag in a DVD+R has a thickness of 10 nm. As canbe seen the jitter is much lower in a broader range of write powers, inparticular for higher write powers compared to the jitter achieved withthe thick reflective layer shown in FIG. 1.

The temperature distribution after applying a dc-power level to aconventional DVD+R recording stack having a thick reflective layer 1made of Ag (100 nm), a dye recording layer 2 (100 nm) and apolycarbonate substrate layer 3 (100 nm) is shown in FIG. 3 a Not shownin FIG. 3 a is the dummy substrate that is glued by means of aUV-curable lacquer on top of the thick reflective layer. Since the dyematerial has very poor heat conduction, the efficient cooling by thereflective layer 1 induces a large temperature gradient near theinterface between the reflective layer 1 and the recording layer 2. Fromthis steep gradient mechanical stress may result which may lead todelamination at the interface between the reflective layer 1 and therecording layer 2.

By reducing the thickness of the reflective layer 1, the cooling becomesless efficient, but advantageously as a consequence the temperaturegradient becomes more gradual as can be seen from FIG. 3 b where thetemperature distribution after applying a DC-power level to a firstembodiment of a record carrier according to the present invention isshown. Here the reflective layer has a thickness of approximately 10 nmand is made of Ag.

The thermal distribution for a second embodiment of a record carrieraccording to the present invention is shown in FIG. 4. Therein, comparedto the record carrier shown in FIG. 3 a, an additional dielectric layer5 made of SiO₂ having a thickness of 20 nm is provided between thereflective layer 1 having a thickness of 40 nm here and the recordinglayer 2. Not shown in FIG. 4 is the dummy substrate that is glued bymeans of a UV-curable lacquer on top of the 40 nm thick reflectivelayer. Further, the recording layer 2 has a reduced thickness of 80 nmwhich leads to a more efficient absorption as is apparent from thesomewhat higher maximum temperature that is reached. The introduction ofthe additional dielectric layer 5 further reduces the temperaturegradient between the reflective layer 1 and the recording layer 2, andthus reduces mechanical stress and prevents delamination.

The thermal distribution for a third embodiment of a record carrieraccording to the present invention is shown in FIG. 5. Therein thereflective layer made of Ag has a thickness of 30 nm which still yieldsrather high reflection, but the thermal capacity is reduced by a factorof 3.3. Nevertheless, also in this embodiment the thermal gradientbecomes less steep. Not shown in FIG. 5 is the dummy substrate that isglued by means of a UV-curable lacquer on top of the thick reflectivelayer.

FIGS. 6 and 7 schematically show two embodiments of record carriersaccording to the invention. FIG. 6 shows an embodiment for which thetemperature profile is shown in FIG. 5. In addition to the three layersshown in FIG. 5, a second polycarbonate substrate layer 4 (about 0.6 mm)is shown, which is glued by means of a UV-curable lacquer on top of thereflective layer 1. FIG. 7 shows an embodiment for which the temperatureprofile is shown in FIG. 4. In addition to the 4 layers shown in FIG. 4,again a second polycarbonate substrate layer 4 (about 0.6 mm) is shown.

The embodiments shown in the figures are to be understood as examples. Anumber of further embodiments and further variations of the thicknessesof the different layers as well as the sequence and the provision offurther layers is possible. The invention provides the advantage to getbroader power margins for write-once optical record carriers, inparticular for DVD+R media. Further, the possibility to go to higherrecording speeds is available. In addition, thinner reflective layerscost less time to sputter, i.e. a faster fabrication is possible and areduction of fabrication costs can be obtained.

1. Write-once optical record carrier comprising: a substrate layer (3);a recording layer (2) of an organic dye material on top of the substratelayer (3); and a metal reflective layer (1) of a thickness below 75 nmon top of the recording layer (2).
 2. Record carrier according to claim1, further comprising a dielectric layer (5) between said recordinglayer (2) and said metal reflective layer (1).
 3. Record carrieraccording to claim 2, wherein said dielectric layer (5) is of athickness below 50 nm, in particular below 25 nm.
 4. Record carrieraccording to claim 1, further comprising a dielectric layer between saidrecording layer (2) and said substrate layer (3).
 5. Record carrieraccording to claim 1, wherein said metal reflective layer (1) issubstantially made of a material of the group consisting Ag, Al, Au, inparticular made of Ag.
 6. Record carrier according to claim 1, whereinsaid metal reflective layer (1) is of a thickness below 50 nm, inparticular below 30 nm.